Update was smooth, without problems after clearing installer cache.
Catchable fatal error: Argument 7 passed to phpbb\template\twig\environment::__construct() must be an instance of phpbb\event\dispatcher_interface, array given, called in /home/..../public_html/..../cache/installer/container_a5fcce7799cdc15359366537322eb9a2.php on line 1967 and defined in /home/..../public_html/...../install/update/new/phpbb/template/twig/environment.php on line 62

While the metadata manager contains changes, this should not affect extensions. If you think this caused extensions to be no longer compatible, then please elaborate on what changed and causes issues for you.

I think we overlooked the event already being part of a previous release and therefore didn't object to the renaming. That is something that can be undone to prevent potential issues for extension authors.

The only non-bc change in Metadata Mgr is the move of the output_metadata_to_template function to acp_extension, where it belongs. And yes, this affects your "upload" extension.
If you find any other non-bc compatible change, it is unintended, and should be fixed.